Various vehicles including land born, airborne and waterborne vehicles are provided with a plurality of antennas for both the transmission and receipt of radio frequency (RF) electro-magnetic transmissions for a variety of purposes. For example, antennas are provided for the receipt of RF transmissions for use in direction finding (DF). In such DF applications, RF transmissions from a signal source are received by the antenna and processed by a receiver and processor for phase and amplitude signature recognition to determine the "direction of the source". Similarly, various transmit antennas are provided for the transmission of RF signals for communication purposes.
A problem associated with the use of antennas on vehicles is that appendages on the vehicle may interfere with the transmit or receipt of RF signals by the antenna. Such interfering appendages include other antennas, armaments, aircraft landing gear, a ship superstructure, etc. In the case of a transmit antenna, these interfering objects may seriously degrade the transmission of RF signals in certain directions relative to the antenna. Therefore, steps are typically taken to test the output transmission at various locations with respect to the antenna after installation on the vehicle to verify that signal transmission is satisfactory in all directions and action is taken to correct an unsatisfactory condition. Similarly, with respect to a receiving antenna, and most particularly a DF antenna, appendages may obstruct or redirect received RF signals such that the receiving antenna does not receive the RF transmission or if it does receive the RF transmission, the indicated direction will be inaccurate because of reflection and redirection.
Even when antennas are installed in optimal position, new antenna installations are typically tested to ensure that the antenna has been properly installed and a signal path is provided between the antenna and the transmitting or receiving equipment on the vehicle.
Another problem associated with the testing of vehicle mounted antennas is that vehicle motion may be indicated as a change in the transmission or receipt of RF energy by the antenna and therefore be improperly interpreted as a problem with the transmission or receipt of RF energy by the antenna. For example, helicopters tend to drift in position when performing a hover turn for antenna pattern measurement. This drift introduces errors into the antenna pattern measurement. These errors would make it extremely difficult to test at night or during other reduced visibility conditions, e.g., fog, rain, etc., because pilots do not have a good visual reference point for the minimization of helicopter drift. Additionally, environmental conditions may cause changes in the position of the test platform. For example, a shift in the wind may cause a change in the position or attitude of a helicopter.